User blog comment:DaniVariAres/Which do u think is easier to get?/@comment-32492558-20171001011122

Mew: 1000 x 4096 = 1/4096000

Ditto: 4096 x 150 Robux = 61440 Robux, but still a 1/4096 chance for the shiny.

Manaphy: 1000 x 4096 = 1/4096000

So, the tie is between Manaphy and Mew, but Manaphy has only been accessible in two events, whereas Mew has been availible ever since the beginning of summer 2017, being accessible basically every day this whole time from its release.